The Irish budget carrier slammed the demands as "unacceptable" and "unnecessary."

Thousands of Ryanair passengers have been affected by the walkout, which started at 03:00 am (0100 UTC). Germany's Cockpit pilots' federation and the Verdi service workers' union said their talks with the Ryanair management had failed to break the impasse.

Last month, Ryanair pilots in five European countries, including Germany, held their first-ever simultaneous walkout, causing some 400 flight cancellations and travel chaos for 55,000 passengers.

Union leaders are expected to announce Thursday details of another coordinated walkout by Ryanair cabin crew in Belgium, Italy, the Netherlands, Portugal and Spain. They have vowed to stage "the biggest strike action the company has ever seen."

Stalled mediation

VC has been demanding better pay and working conditions for Ryanair workers for months.

The Irish airline insists the German pilots' dispute should be settled by an Irish mediator, but VC maintains he or she "could not possibly have enough knowledge of German law and therefore already is not qualified to act as an arbitrator in this case."

The union has called the strike from Wednesday, September 12, 3:01 a.m. (0101 UTC) to Thursday, September 13, 2:59 a.m. (0059 UTC).

Ryanair is currently experiencing the worst wave of strikes in its history. While the low-cost carrier has begun recognizing pilot and cabin crew unions across Europe for the first time, workers are unhappy that improvements have yet to be made on pay and other conditions.

"It is unacceptable that a union representing Ryanair's German pilots, who earn up to 190,000 euros ($220,000) a year and work a five-day week followed by a four-day weekend, is now threatening customers travel plans at short notice," said Jacobs.

The majority of Ryanair pilots based in Germany are direct employees, unlike the situation in other Ryanair markets, and in theory, a widely implemented strike of German-based pilots could significantly affect the airline's operations.

Ryanair said it would would be forced to cancel 150 out of 400 scheduled flights to and from Germany on Wednesday, but it would try to minimise the disruption for customers.

Separately, pilots unions across Europe on Friday threatened "the biggest strike action the company has ever seen" over their members' working conditions. The strike, so far organised by unions in Belgium, Holland, Italy and Spain, is scheduled for the last week in September. The walk-out could be called off should an annual general meeting of Ryanair shareholders on September 20 meet union demands. Ryanair has banned the media from the AGM and there will be no news briefing afterwards, according to a statement cited by Irish public broadcaster RTE.

Air traffic booming around the world

Crowded skies

More passengers mean more aircraft in use. Asia's fleet for instance is set to be twice as big by 2035 to total about 17,000 planes. North America will have some 9,800 aircraft and Europe's about 7,900 (up from 4,610 today).

Air traffic booming around the world

Pilots in high demand

More planes in use mean more pilots are needed. Boeing reckons that about 617,000 new pilots will have to be recruited by 2035, particularly in Asia. On top of that, there's a need for 679,000 new maintenance staff and 814,000 additional flight assistants, says Boeing. Airbus sees a need for 560,000 new pilots.

Air traffic booming around the world

The biggest hubs

Europe's biggest hubs in terms of passenger numbers are London Heathrow, Paris-Charles De Gaulle and Frankfurt in Germany, handling between 73 million and 64 million passengers. In Frankfurt, over 2 million tons of cargo were dealt with in 2014. The world's largest airport in Atlanta in the US boasted 100 million passengers in 2015.

Air traffic booming around the world

Budget airlines to the rescue

Growth at German aviation hubs is not so much a result of booming German carriers. It's more about the strength of foreign airlines. The number of German carriers' take-offs has shrunk steadily over the past six years. By contrast, no-frills competitors such as Ryanair and Easyjet have seen a 14-percent rise in take-offs from German airports.

Air traffic booming around the world

Largest strike action in history

German flagship carrier Lufthansa was able to log 1.8 billion euros ($1.94 billion) in profit last year, beating results from a year earlier and despite a series of strikes crippling the airline. Industrial action organized by the VC pilots' union since 2014 has cost Lufthansa about half a billion euros. Meanwhile, management and pilots have struck a deal in principle.

Air traffic booming around the world

The biggest earners

Aviation is a lucrative business, but some market players do better than others. While airlines tend to get a 4-percent yield on their invested capital, German airport operators and plane manufacturers secure between 6 and 7 percent. Doing even better are air traffic control, haulers and booking services, which - says a McKinsey study - can get as much as 20 percent in proceeds from capital.

Air traffic booming around the world

What's in store for us?

Here you are: beds for tired travelers, massagers and generous legroom, childcare services and a bar plus shower cabinets to make you feel alright. This is what passengers hope they will see on board in the not-too-distant future. Should this become a reality, passenger numbers are bound to soar further.